Conventionally, in order to screen out defective products produced in the early stages of manufacturing processes, accelerated tests are performed on semiconductor devices and wafers by operating the semiconductor devices and wafers at high temperatures and high voltages. Such operations are called burn-in.
In recent years, techniques for collectively performing burn-in at a wafer level (hereinafter, will be referred to as wafer-level burn-in) have been frequently performed. In wafer-level burn-in, tests are conducted by operating devices while inputting high voltages and signals to the power supply electrodes and the plurality of input/output electrodes of the devices.
On the other hand, in semiconductor ICs, as tests become complicated and the number of pins increases with higher levels of integration achieved by finer processes, DFT (Design for Test) techniques have been developed as circuit designs for facilitating tests. BIST (Built-in self test) circuits have been also installed as self-test circuits.
These test facilitating techniques greatly contribute to a reduction in the number of pins in wafer-level tests. In recent years, in combination of these techniques with probing techniques for wafer-level burn-in, attempts have been made to collectively perform wafer tests or tests on package conditions at a wafer level.
Referring to FIG. 5, a configuration for DFT in conventional wafer-level burn-in will be described below (for example, see Japanese Patent Laid-Open No. 2000-227458).
FIG. 5 is a circuit diagram showing the configuration for DFT in a conventional semiconductor IC. In FIG. 5, reference numeral 501 denotes burn-in mode selector circuits, reference numeral 502 denotes scanning flip-flop circuits, reference numeral 503 denotes combinational circuits, reference numeral 504 denotes a pseudorandom number generator circuit, reference numeral 505 denotes an output decision circuit, reference numeral 507 denotes a reset terminal fed with a reset signal RS from the outside, reference numeral 508 denotes a scan shift/capture switching terminal, and reference numeral 509 denotes a burn-in mode setting terminal for setting the switching of the burn-in mode selector circuits 501. The clock input terminals of the scanning flip-flop circuits 502 are fed with the same clock (not shown).
The semiconductor IC of FIG. 5 has first to m-th scan chain holding sections having the plurality of combinational circuits 503 and the plurality of scanning flip-flop circuits 502. Reference numerals S1, S2, . . . , Sm respectively denote the outputs of the scanning flip-flop circuits 502 of the final stages in the first, second, . . . , m-th scan chain holding sections.
In the conventional example, the pseudorandom number generator circuit 504 is designed in the first scan chain holding section. As shown in FIG. 5, an output signal f generated in the pseudorandom number generator circuit 504 is supplied through the burn-in mode selector circuits 501 to the inputs of the scanning flip-flop circuits 502 in the first stages of all the scan chain holding sections.
With this configuration, it is possible to supply random numbers to all the scan chain holding sections and apply a proper stress to the overall circuit during a burn-in test. Further, pseudorandom numbers generated inside eliminate the need for scan input from the outside, so that the semiconductor IC can be operated with a small number of terminals.
The output decision circuit 505 is specifically configured as shown in FIG. 6. The output decision circuit 505 monitors, for each fixed period of time, the output signals S1, S2, . . . , Sm of the scanning flip-flop circuits 502 in the final stages of the scan chain holding sections, and the output decision circuit 505 outputs decision signals U1, U2, . . . Um as output signals indicating the states of the circuit when a stress is improperly applied or the circuit fails and malfunctions.
At comparisons in EX-NOR circuits 616, 617 and 618, expected values M11 to M23 are set and the expected values of the output signals S1 to Sm are calculated and set beforehand by simulation.
According to the conventional example, when a plurality of scan chains are configured to make a scan design, one of the scan chain holding sections has the pseudorandom number generator circuit 504 and the output signal f of the pseudorandom number generator circuit 504 is supplied to the inputs of the scanning flip-flop circuits 502 in the first stages of all the scan chain holding sections, so that random numbers can be supplied to all the scan chain holding sections and a proper stress can be applied to the overall circuit during a burn-in test.
Further, since the output decision circuit 505 is provided, it is possible to decide whether or not a stress is properly applied and the circuit normally operates without failures during a burn-in test, improve the reliability of a reliability test, and prevent a defective chip from being brought as it is to the subsequent step.
In the output decision circuit 505 of the conventional example, as shown in FIG. 6, the decision signals U1 to Um are outputted from decision signal output terminals 620 to 622, respectively. The decision signal output terminals 620 to 622 may not be provided. As shown in FIG. 7, the decision signals U1 to Um outputted from flip-flop circuits 615 may be inputted to an AND circuit 703 and the output of the AND circuit 703 may be outputted as a decision signal U from a single decision signal output terminal 704.
In this case, when the decision signal U is “H”, it can be decided that a stress is properly applied and circuits making up the first to m-th scan chain holding sections normally operate without failures. When the decision signal U is “L”, it can be decided that a stress is not properly applied or a failure occurs in the circuits making up the first to m-th scan chain holding sections. Instead of the AND circuit 703, a NAND circuit may be provided. In this case, the decisions about “H” and “L” of the decision signal U are reversed.
Further, wafer-level burn-in is characterized in that a wafer is contacted and operated in a collective manner. However, some burn-in devices have just a few comparators for receiving decision signals. Thus in many cases, a circuit connected via a tristate buffer 705 having an output control signal 800 from an output control terminal 707 as an input is installed as a circuit for controlling an output, in order to enable a tester to receive the decision signals in a plurality of times even when just a few comparators are provided.
Referring to FIG. 8, the following will describe a conventional method for performing a burn-in test simultaneously on a wafer including the semiconductor IC. A technique of simultaneously making contact with a wafer has been realized by Matsushita Electric Industrial Co., Ltd. as probes and the like having a three-layer structure for simultaneously making contact with the wafer. In FIG. 8, it is assumed that two semiconductor ICs C1 and C2 of a plurality of semiconductor ICs configured on a wafer are simultaneously operated.
In FIG. 8, reference numeral 507 denotes the reset terminal of FIG. 5, reference numeral 508 denotes the scan shift/capture switching terminal of FIG. 5, reference numeral 509 denotes the burn-in mode setting terminal of FIG. 5, and reference numeral 707 denotes the output control terminal fed with the output control signal 800 for controlling the output of the tristate buffer 705 of FIG. 7. The output of C1 is controlled by an output control signal 800-1 inputted to the output control terminal 707 on the side of C1, and the output of C2 is controlled by an output control signal 800-2 inputted to the output control terminal 707 on the side of C2.
Reference numerals 801 to 804 denote signals connected to the two semiconductor ICs in a shared manner. Reference numeral 801 denotes a burn-in mode setting signal connected to the burn-in mode setting terminals 509 of C1 and C2 and supplied from the tester to a wafer 806 as a signal for setting the burn-in modes of the semiconductor ICs. Reference numeral 802 denotes a scan shift/capture switching signal connected to the scan shift/capture switching terminals 508 of C1 and C2 and supplied from the tester to the wafer 806 as a signal for setting the scanning operations of the semiconductor ICs. Reference numeral 803 denotes a reset signal connected to the reset terminals 507 of C1 and C2 and supplied from the tester to the wafer 806 as a signal for initializing the semiconductor ICs. Reference numeral 804 denotes a reference clock connected from the corresponding terminals of C1 and C2 to the clock input terminals of the scanning flip-flop circuits in C1 and C2. The reference clock 804 is supplied from the tester to the wafer 806 as a reference signal of the operation timing of the semiconductor ICs. Reference numeral 805 denotes a pass/fail output signal connected to output terminals 706 (see FIG. 7) of C1 and C2. The pass/fail output signals 805 are transferred from the wafer 806 to the tester as signals indicating decision results respectively outputted from C1 and C2.
With this configuration, the plurality of semiconductor ICs on the wafer can be collectively operated in response to the signals 801 to 804.
Further, in order to confirm whether or not C1 has been correctly operated, the output control signal 800-1 to the output control terminal 707 is initially controlled such that the output control signal 800-1 to the output control terminal 707 for C1 is enabled. At this moment, the output control signal 800-2 is controlled such that the output control signal 800-2 to the output control terminal 707 for C2 is disabled. After an initial reset in response to the reset signal 803 to the reset terminals 507 in the signal conditions of the output control signals 800-1 and 800-2 to the output control terminals 707, the reference clock 804 is supplied and a burn-in test is conducted. The results of the burn-in test are collected in the AND circuit of FIG. 7 and outputted after a decision in the output decision circuit 505.
Thus it is possible to confirm whether or not C1 has been correctly operated.
Further, in order to confirm whether or not C2 has been correctly operated, the output control signal 800-2 to the output control terminal 707 is initially controlled such that the output control signal 800-2 to the output control terminal 707 for C2 is enabled. At this moment, the output control signal 800-1 is controlled such that the output control signal 800-1 to the output control terminal 707 for C1 is disabled. After an initial reset in response to the reset signal 803 to the reset terminals 507 in the signal conditions of the output control signals 800-1 and 800-2 to the output control terminals 707, the reference clock 804 is supplied and a burn-in test is conducted. The results of the burn-in test are collected in the AND circuit of FIG. 7 and outputted after a decision in the output decision circuit 505.
In the conventional example, the wafer made up of the two semiconductor ICs was described. For n semiconductor ICs, the foregoing contents are repeated n times to confirm operations on the overall wafer.
As described above, as a flow of confirming whether an operation is correct or not, as shown in FIG. 10, the following is repeated n times: output control is enabled for a semiconductor IC to be confirmed, an operation test is conducted, and then a decision is read, so that operations on the overall wafer are confirmed.
In the conventional example, the two semiconductor ICs C1 and C2 are connected to single wiring for outputting the pass/fail output signal 805 from the output terminal 706. In reality, semiconductor ICs are configured in a plurality of rows and columns on a wafer and thus generally, a plurality of rows and columns (m, M) shown in FIG. 9 are connected via wiring.
As described above, with the configuration of the conventional semiconductor ICs, operations can be performed and confirmed with a small number of terminals, so that a wafer burn-in test can be easily conducted simultaneously.
The conventional test facilitating technique greatly contributes to a reduction in the number of pins in wafer-level tests. In recent years, in combination with probing techniques for wafer-level burn-in having been applied to mass production, attempts have been made to collectively perform wafer tests or tests on package conditions at a wafer level.
However, in the foregoing semiconductor ICs, expected values are set inside for the output decision circuit for deciding whether the semiconductor IC is acceptable or not. Thus in the event of wrong simulation results during the design of the semiconductor IC, redesign is necessary.
Moreover, during wafer burn-in, operations have to be performed not in a collective manner but in blocks to confirm the operations. Further, the semiconductor IC has to be operated for each decision, resulting in a long test time.